What Is a Rack Unit?

A rack unit is the unit of height used for equipment that mounts in a standard 19 inch (19″) rack. One rack unit, written 1U or sometimes 1RU, is 1.75 inches, or 44.45 mm. Equipment heights are always whole or half multiples of it, so a 1U server, a 2U storage chassis and a 4U GPU system all fit the same rails and stack without gaps.

The dimension comes from the EIA-310 standard in the US and the equivalent IEC 60297 internationally. Both fix the 19 inch panel width and the 1.75 inch vertical pitch, which is why equipment from different manufacturers fits the same rack.

A front panel is made very slightly shorter than its nominal height, typically by about 1/32 inch (0.8 mm), so that a 1U panel measures around 43.7 mm. The small gap lets neighbouring equipment be fitted and removed without binding.

Converting is simple: multiply the number of U by 1.75 for inches or by 44.45 for millimetres. The table shows common heights for equipment and for whole racks.

The rack heights are the usable mounting space. The finished cabinet is taller, because the frame, top and plinth add to it: a 42U cabinet usually stands about 2 m high. Rack-mount monitors and panel PCs also take a whole number of U, which the datasheet states.

A 19 inch rack is the standard frame or cabinet for mounting servers, network switches, monitors, power supplies and other equipment, named after the 19 inch (482.6 mm) width of the front panels it takes. The format comes from telephone exchanges: AT&T standardised panels of that width on a 1.75 inch height step in the early 1920s, and the same dimensions carried over into broadcast, test and measurement, and later data-centre equipment. Today they are defined by EIA-310-E in the US and IEC 60297-3-100 internationally, which is why a 19 inch chassis from one maker fits a rack from another.

The 19 inch figure is the width of the equipment, not of the rack. Racks come as open two-post and four-post frames, enclosed cabinets with doors and side panels, wall-mount cabinets, and transit cases with internal shock mounts for vehicles and field use, and all of them take the same 19 inch equipment. In telecoms you may also meet the 23 inch rack, which keeps the same rack unit but takes wider panels, so 19 inch equipment needs adapter brackets to fit it.

Much industrial equipment is built to the format too, from rack mount panel PCs for operator consoles to rack mount industrial Ethernet switches for control and substation networks.

Each rack unit has three mounting holes on each vertical rail. The holes are not evenly spaced: the pattern repeats at 0.625, 0.625 and 0.5 inches (15.875, 15.875 and 12.7 mm), so the closer pair marks the boundary between one U and the next. Many racks print U numbers on the rails to make counting easier, and it is good practice to record each item’s position by its lowest U so equipment can be found and replaced quickly.

The 19 inch rack is by far the most common, but you may meet a few others. Ten-inch racks are half-width versions for small networks and keep the same 1.75 inch rack unit. ETSI racks used in European telecoms are 535 mm wide and count height in 25 mm steps. The Open Compute Project’s Open Rack uses a 21 inch equipment width and its own 48 mm OpenU. Equipment for one format does not fit another without adapters, so check the rack before choosing the chassis.

Counting U is only the start. Power distribution, cooling and weight usually limit a rack before space does. Most rack equipment draws cool air in at the front and exhausts it at the back, so fill unused U with blanking panels to stop hot air circulating back to the front. Leave spare U for cable management and growth, and put the heaviest equipment at the bottom. In factories and substations, check the rack’s depth and the ambient temperature as well, since control-room and cabinet racks are often shallower and warmer than data-centre ones.

Our rack mount servers cover the common heights, from the Premio LLM-1U-RPL 1U edge AI servers to the Premio LLM-2U-AM5 2U server and Premio LLM-3U-AM5 3U server with room for full-height GPU cards. For the operator’s view, 19 inch rack mount monitors fit the same rails, and our rack mount KVM consoles fold a screen and keyboard into a 1U drawer.

We build rack-mount systems to fit the cabinet on site as standard, fitting the rails, storage, memory and expansion cards and checking the depth, power and airflow before the system ships.